The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP Third Generation Partnership Project        ANR Automatic Neighbor Relation        BW Bandwidth        CA Carrier Aggregation        CC Component Carrier        CDM Code Division Multiplexing        CRS Cell Specific Reference Signal        DCI Downlink Control Information        DL Downlink        DRX Discontinuous Reception        eICIC Enhanced Inter-Cell Interference Coordination        E-UTRA Evolved Universal Terrestrial Radio Access        eNB Evolved Node B/Base Station in an E-UTRAN System        E-UTRAN Evolved UTRAN (LTE)        FDD Frequency Division Duplex        FDM Frequency Division Multiplexing        IE Information Element        ISM Industrial, Scientific, Medical        L3 Layer 3 (network layer)        LTE Long Term Evolution        LTE-A Long Term Evolution Advanced        MAC Medium Access Control        MAC CE MAC Control Element        MGRP Measurement Gap Repetition Period        PDCCH Physical Downlink Control Channel        PDSCH Physical Downlink Shared Channel        PCell Primary Cell        PHY Physical Layer        PUCCH Physical Uplink Control Channel        PUSCH Physical Uplink Shared Channel        RAT Radio Access Technology        RNTI Radio Network Temporary Identity        RRC Radio Resource Control        RRM Radio Resource Management        Rx Reception, Receiver        SCell Secondary Sell        SFN System Frame Number        SI System Information        TA Timing Advance        TD Timing Delay        TDD Time Division Duplex        TDM Time Division Multiplexing        TTI Transmission Time Interval        TTT Time To Trigger        Tx Transmission, Transmitter        UE User Equipment        UL Uplink        UTRAN Universal Terrestrial Radio Access Network        
LTE-A (Long Term Evolution Advanced) aims at providing significantly enhanced services by means of higher data rate and lower latency with a reduced cost. Carrier Aggregation (CA) is one of key technologies to greatly improve the data rate. In Release 10 of 3GPP, the current discussion is mainly about CA of either FDD-FDD system or TDD-TDD system on licensed LTE bands, which are scarce resources.
Unlicensed band utilization for LTE systems is a subject of recent research as it may offer an operator an opportunity to offload traffic from a crowded licensed LTE band to an unlicensed band if necessary. This is likely to be considered in Release 12 of 3GPP and beyond releases of 3GPP documents. By using the unlicensed band and licensed band CA, we can get some valuable benefits such as balancing the traffic load when necessary, improving the peak data rate, and improving the spectrum efficiency in general for the operator.
The available unlicensed bands are different in different geographical locations around the world. The most commonly considered unlicensed spectrum is 2.4 GHz ISM band with BW of 83.5 MHz, the 5 GHz unlicensed band with BW larger than 100 MHz, and White Space spectrum which comprises unused parts (bands) of TV spectrum in the 54-698 MHz range (now only US). However, quite a few challenges for the unlicensed band need to be resolved before efficient utilization can be made. One critical issue is the unlicensed band's inherent “share” property. For example for ISM bands, there may be WIFI systems (IEEE 802.11 a/b/g/n/ac), as well as BLUETOOTH, ZIGBEE, WIRELESSUSB, CORDLESS PHONE, MICROWAVE, etc. As for White Space spectrum bands, there may be some other systems also trying to use those spectrum bands. In order to deploy the LTE system on the unlicensed band, an eNB will need to turn off its use of unlicensed bands from time to time due to a large interference or due to giving another system a chance to transmit. An irregular ON/OFF mechanism of LTE system is described in PCT Application PCT/CN2011/001167 filed Jul. 14, 2011.
Before configuring a SCell to a UE, it is important that the eNB knows whether or not the concerned UE is in the coverage of the corresponding carrier. Currently, in the LTE system, the UE will do RRM measurement on any configured measurement object, but the choice of which object to measure is up to a UE implementation. Since the LTE system may have an irregular ON/OFF mechanism on the unlicensed band, leaving the measurement object choice purely to the UE may have some problem because during the OFF period, the UE may not be able to detect the CRS which may impact accuracy of the measurement and cause detection errors.
In the legacy LTE system, measurement gaps are configured to let UE to do measurement for other (inter-frequency) neighbor cells or other RATs. Once configured, the measurement gap happens periodically and the UE can do measurements of the neighbor cells it selects using the same gap pattern. In other words, during the measurement gaps, the UE performs its inter-frequency and inter-RAT measurements and it depends on the UE implementation which measurement object is measured at which particular measurement gap. Thus, it is difficult for the network to control the measurements for the unlicensed band using the conventional measurement gaps to ensure that the measurements would only happen during the LTE ON period. A more flexible solution is needed.
In the current LTE systems, there are two types of measurements: one type requires the measurement gaps and the other type doesn't require these gaps. The UE reports the gap requirement to the eNB with UE capability signaling and it is up to the eNB to configure the gaps if the corresponding measurement is needed. In the Stage 2 specifications 3GPP TS 36.300 V10.4.0 (2011-06), there are example schematics illustrating some scenarios on whether the measurement gap is needed as shown in FIG. 1 (where the current and target cells operating at the same carrier frequency) or the measurement gap is not needed as shown in FIG. 2 (where the current and target cells operating at different carrier frequencies). These
A measurement gap could be configured to trigger the UE to do inter-frequency and inter-RAT measurement. The UE behavior is described in RRC specifications 3GPP TS 36.331 V10.2.0 (2011-06) as follows:
The UE shall: 1> if measGapConfig is set to setup:2> if a measurement gap configuration is already setup, release  the measurement gap configuration;2> setup the measurement gap configuration indicated by the  measGapConfig in accordance with the received gapOffset,  i.e., each gap starts at an SFN and subframe meeting the  following condition:SFN mod T = FLOOR(gapOffset/10);subframe = gapOffset mod 10;  with T = MGRP/10 as defined in TS 36.133 [16]; 1> else:2> release the measurement gap configuration;
Furthermore, the measurement gap can have two possible periodicity configurations, as described in RRM requirements specification 3GPP TS 36.133 V10.3.0 (2011-06), as shown in Table 1 below.
TABLE 1Measurement gap patterns specified in 36.133Minimum available time forinter-frequency and inter-Measurement GapRAT measurements duringGap PatternMeasurementGapRepetition Period480 ms periodIdLength (MGL, ms)(MGRP, ms)(Tinter1, ms)Measurement Purpose064060Inter-Frequency E-UTRANFDD and TDD, UTRAN FDD,GERAN, LCR TDD, HRPD,CDMA2000 1x168030Inter-Frequency E-UTRANFDD and TDD, UTRAN FDD,GERAN, LCR TDD, HRPD,CDMA2000 1x
The measurement gaps happen periodically once configured, and the eNB cannot control which measurement object is measured during each gap.
Additionally, a method called enhanced inter-cell interference coordination (eICIC) has been defined in Release 10 of 3GPP. In this method, the eNB is able to affect slightly more specifically the measurement instants when the UE does intra-frequency measurements. However, this mechanism so far (up to Release 10 of 3GPP) is only possible for the intra-frequency measurements (which never require measurement gaps), so is not directly applicable to the measurements requiring measurement gaps. Moreover, the eICIC does not provide a certainty of when the UE does the measurements, and the 40 ms periodicity that the eICIC patterns have may limit its usefulness to the unlicensed band usage.
The Release 8 of 3GGP for the LTE includes a measurement mechanism called Automatic Neighbor Relation (ANR), see 3GPP TS 36.300 V10.4.0 (2011-06) and 3GPP TS 36.331 V10.2.0 (2011-06) for description of the procedure, wherein the eNB instructs the UE to do measurements in a “best effort” basis for a specific measurement purpose, i.e., acquiring the SI from a neighbor cell, a procedure which is normally not supported during active transmission. The measurement is given with a timer attached to control how long the UE does the measurement. The idea is that the UE is expected (to the best of its ability) to use idle periods in transmission (e.g., DRX, unused measurement gaps) for this specific measurement purpose, with the limitation that the measurement should not cause degradation to the normal UE operation. Once the UE has either completed the specific measurement purpose (i.e., has acquired the required SI information from the specified neighbor cell) or the timer attached to the specific measurement purpose expires, the UE automatically reports the results of this procedure to the eNB, i.e., letting the eNB know whether the measurement was successful and if it was, what the results were. After this, the specific measurement purpose is removed until the eNB chooses to re-configure it. However, it should be noted that the ANR procedure places no restrictions as to when the UE does the measurement.